Cannot access memory at address

Hi! I have a trauble which seems to be stack trauble - but i am not sure... Why i think this is stack? because where in the gdb shows no stack trace and only says Cannot access memory at address the code runs about 100 pthreads, uses select and socket IO. So, my pthreaded code crashes by

Execution continues after a "Cannot access memory at address 0x......" message

s Cannot access memory at address 0x2f6c6962. (gdb) bt #0  0x7fbbaa48 in ?? () #1  0x7fbb4310 in ?? () #2  0x7fbbedc4 in ?? () #3  0x7fbbee5c in ?? () #4  0x7fbbefa4 in ?? () #5  0x7f6c9844 in SO_per_src_lookup () from /usr/lib/libc.so.1 #6  0x7f6c8108 in nss_get_backend_u () from /usr/lib/libc.so.1 #7

why "Cannot access memory at address 0x...."

I'm debugging some multi-threaded server program. Sometimes gdb says 'Cannot access memory at address 0x.' even though, I didn't give any command except 'run'. I guess it differs from my program's "segmentation fault". Why